Screw back-out prevention mechanism

ABSTRACT

A screw back-out prevention mechanism for a bone fixation system is provided. The mechanism, when engaged, either locks or retains the screws of the bone fixation system in place thereby preventing the screws of the bone fixation system from backing out of the bone, and in turn reducing the risk of device separation or failure in the bone fixation system.

CROSS-REFERENCE TO RELATED APPLICATIONS

The current application is a continuation application of U.S.application Ser. No. 12/180,787 filed Jul. 28, 2008, now U.S. Pat. No.8,177,821, which application claims priority to U.S. ProvisionalApplication No. 60/952,161, filed Jul. 26, 2007, the disclosure of whichis incorporated herein by reference.

FIELD OF THE INVENTION

The current invention is directed to a mechanism for preventing theback-out of a screw; and more particularly for a mechanism forpreventing the back-out of screws in a bone fixation device.

BACKGROUND OF THE INVENTION

The use of fixation plates for the treatment of bone fusions andfixations has grown more prevalent over the past decade. Indeed, whileearly procedures using fixation plates were generally restricted to longbones and lower lumbar levels of the spine, fixation plates haveincreasingly found applications in other bone instrumentation such as inthe cervical spine.

A typical bone fixation plate is provided with a plurality of borestherethrough. A corresponding plurality of fastener members, typicallybone screws having a headed portion and a threaded shaft, are providedto secure the plate to the bone, or bones, to be fixated. A commonproblem with the use of fixation plates, regardless of their location,is the tendency of the bone screws to “back-out” of the underlying bone.This problem is particularly prevalent in areas of high stress such asthe spine. Given the delicate nature of the spine, anything that mayresult in post-operative complications, such as plate movement orrevision, can seriously endanger the patient's long-term prognosis.

Bone fixation systems have employed various techniques in an attempt toovercome the problem of screw back-out. Current techniques rely eitheron the use of specially designed bone screws, are irreversible, orrequire special procedures that could complicate the surgery. Forexample, U.S. Pat. No. 5,275,601 discloses a self-locking bone fixationsystem wherein the heads of the bone screws are frustoconical in shapeand have a directionally corrugated outer surface; U.S. Pat. No.5,269,784 discloses a threaded screw nut that threadingly engages aportion of the bone screw to thereby secure the bone screw to thefixation plate; U.S. Pat. No. 4,484,570 discloses a bone fixation systemwherein the heads of the bone screws are hollow and expandable; and U.S.Pat. No. 5,578,034 discloses a bone fixation system in which the platesare heated after insertion, thereby expanding a retaining mechanism intoplace around the screw.

All of the cited prior art systems suffer from one or more undesirabledrawbacks. First, some of these prior art systems rely on a retainerthat itself uses a threaded connection to maintain the bone screws inposition, meaning that the problem of screw back-out still exists.Second, several of these systems permanently seal the screw into place,rendering revision or alteration of the plate very difficult. Finally,the requirement that one use a particular, specially designedproprietary bone screw to prevent back-out limits a surgeon's ability tochoose the best-engineered screw for a particular application becausethe proprietary bone screw may have inappropriate specifications such asthread pitch. Accordingly, a bone fixation system incorporating amechanism for preventing screw back-out that is simple to use and reviseand can be operated with any standard bone screw would be desirable.

SUMMARY OF THE INVENTION

The current invention is generally directed to a screw back-outprevention device. The device generally comprises at least one moveableelement and a rotary activation element that interlockingly engages suchthat the rotation of the rotary activation element causes the at leastone moveable element to advance into a lock position where the back-outof the screw is prevented.

In one embodiment of the screw back-out prevention device of the currentinvention the moveable plates and the rotary element are engagedtogether via an interlocking pin and curvilinear slot.

In another embodiment the screw back-out prevention device of thecurrent invention operates in cooperation with a bone fusion device. Insuch an embodiment, the device may either be integrated into the body ofthe fusion device or attached to a top surface of a bone fusion device.

In still another embodiment of the screw back-out prevention device ofthe current invention the rotary activation element is removably engagedto the at least one moveable plate.

In yet another embodiment the screw back-out prevention device of thecurrent invention operates by covering the exposed surface of the headof the screw when in the lock position.

In still yet another embodiment the screw back-out prevention device ofthe current invention operates by engaging a portion of the screw in thelock position. In such an embodiment, the device of the currentinvention either directly or indirectly engages the screw. In theembodiment of the current invention where the device indirectly engagesthe screw it may do so through a retention ring that may either beintegrated into a screw or disposed within the bone fusion plate itself.

In still yet another embodiment the screw back-out prevention device ofthe further invention operates through at least one cam designed torotate into locking engagement with at least a portion of the screw.

In still yet another embodiment, the screw back-out prevention device ofthe current invention is formed from a material selected from the groupconsisting of Ti, stainless steel and NiTi.

BRIEF DESCRIPTION OF THE FIGURES

The description will be more fully understood with reference to thefollowing figures, which are presented as exemplary embodiments of theinvention and should not be construed as a complete recitation of thescope of the invention, wherein:

FIGS. 1 a and 1 b provide schematic diagrams of a screw back-outprevention device in accordance with the current invention;

FIGS. 2 a and 2 b provide top and side view schematic diagrams of aspinal fixation plate incorporating a screw back-out preventionmechanism in accordance with one exemplary embodiment of the currentinvention;

FIG. 3 shows schematic diagrams of the individual components of thescrew back-out prevention mechanism shown in FIGS. 2 a and b;

FIGS. 4 a and 4 b provide top and side view schematic diagrams of aspinal fixation plate incorporating a screw back-out preventionmechanism in accordance with one exemplary embodiment of the currentinvention;

FIG. 5 shows schematic diagrams of the individual components of thescrew locking/retention mechanism shown in FIGS. 4 a and b; and

FIGS. 6 to 9 provide schematic diagrams of the various mechanisms ofoperation of several exemplary embodiments of the screw back-outprevention mechanism in accordance with the current invention.

DETAILED DESCRIPTION OF THE INVENTION

The current invention is directed generally to a mechanism forpreventing screw back-out in a bone fixation system. The back-outprevention device of the current invention uses a plurality of moveablecomponents to directly or indirectly engage the screws of the bonefixation system to either lock or retain the screws into a specificposition within the overall bone fixation system.

A generic schematic of the screw back-out prevention device is providedin FIG. 1. As shown, the device (10) generally comprises at least onemoveable component (12) that can be compelled either directly orindirectly to lock or retain the screw (14) into a desired position bythe operation of a second rotary activation element (16).

An exemplary embodiment of the screw back-out prevention system of thecurrent invention is shown schematically in FIGS. 2 and 3. In this firstexemplary embodiment, the device (20) comprises two moveable plates (22)and a rotary component (24) for engaging and moving the moveable platesinto and out of screw retention/locking alignment. In the embodimentshown best in FIG. 3, the plates (22) have a pair of pins (26) that aredesigned to engage a pair of cooperative curvilinear slots (28) on therotary component (24). As shown in FIGS. 2 a and 2 b, when the rotaryelement (24) is rotated the curvilinear slots (28) apply a force to thepins (26), which in turn direct the movement of the plates. Thedirection and distance of the movement of the plates (22) are controlledby the shape of the curvilinear slots (28) and the direction of rotationapplied to the rotary component (24). As shown in FIGS. 2 and 3, in thecurrent embodiment the counter-clockwise rotation of the rotarycomponent (24) imparts a motion in the outward direction to the moveableplates (22). It should be understood, however, that the directionimparted can be reversed without altering the function of the currentinvention.

Although the embodiment shown in FIGS. 2 and 3 are designed such thatthe rotary component (24) is slotted and the moveable plates (22) havepins, an opposite arrangement, as shown in FIGS. 4 and 5 would beequally acceptable. Specifically, as shown in FIG. 5, in a secondexemplary embodiment of the device the pins (26) are disposed on therotary component (24) and the curvilinear slots (28) are disposed on themoveable plates (22). Moreover, although only single pin/slotcombinations are shown, it should be understood that any number of pinsmay be made to engage the curvilinear slots of the rotary component.

FIGS. 1 to 5 show a number of schematic diagrams of exemplaryembodiments of the back-out prevention device of the current inventionand the arrangement of the main components thereof; however, it shouldbe understood that the manner in which the moveable components of thedevice prevent screw back-out may themselves take a number of differentforms. Schematic diagrams showing exemplary mechanisms are provided inFIGS. 6 to 9.

FIGS. 6 a and 6 b provide schematic diagrams showing the mechanism ofoperation of a first embodiment of the screw back-out prevention deviceof the current invention. In this embodiment the moveable plate (30) ispositioned relative to the screw (32) such that when moved into thescrew retention position (FIG. 6 b) the plate covers the head of thebone screw, thereby preventing the screw from backing out of the bonefixation pate.

FIGS. 7 a and 7 b provide schematic diagrams of the mechanism ofoperation of a second embodiment of the screw back-out prevention deviceof the current invention. In this embodiment, the moveable plate (34)directly engages an interlock portion of the screw (36) specificallydesigned for such a function.

FIG. 8 provides a schematic diagram of the mechanism of operation of athird embodiment of the screw back-out prevention device of the currentinvention. In this embodiment, the moveable plate (40) engages aretention ring (42) positioned either on the screw itself or within thebone fixation plate. The retention ring (42) then directly engages aninterlock portion of the screw (44) specially designed for such afunction.

Although in the embodiments shown in FIGS. 7 and 8 the interlock portionof the screws (36 & 44) take the form of slots designed to interact withthe moveable plate or retention ring, it should be understood that theengagement portion may take any shape or form suitable to receive aportion of the moveable plate thereby securing the screw into position.Examples of suitable engagement portions may include, for example, ahole and pin.

Finally, FIG. 9 provides a schematic diagram of the mechanism ofoperation of a fourth embodiment of the screw back-out prevention deviceof the current invention. In this embodiment, the moveable plate (46)engages a cam mechanism (48) that rotates when activated by the moveableplates to engage a portion of the screw (50) thereby locking the screwinto place.

Regardless of the actual design of the screw back-out prevention deviceand its manner of operation, it should be understood that a furtherlocking mechanism may be included in its design to ensure that onceengaged in the locked position the moveable elements cannot beinadvertently moved out of said position. In a preferred embodiment, thelocking mechanism is designed to be reversibly engaged such that theback-out prevention device can be unlocked if necessary, such as, forexample, for revision or removal of the bone fusion system. However,such a locking mechanism may take any suitable form, including, forexample, a blocking element inserted between the moveable plates toprevent movement in the reverse direction, a set screw, pin, etc.

Although all of the above figures and the accompanying descriptive text,have focused only on the arrangement of the main components of the screwback-out prevention device of the current invention, it should beunderstood that the device can either be integrated into a bone fixationplate, such as a spinal fusion plate, or attached to the top of a platesuch that the it can retain or lock the screws of the bone fixationplate into place. If the screw back-out prevention device is attached tothe top of a bone fixation plate, the attachment may be either permanentor temporary, i.e., the device may be integrally affixed to the platesuch as by welding, or can be removably attached such as by screws orother removable fasteners. In addition, the rotary component may eitherbe permanently integrated with the moveable plates, or may be a separatetool that is engaged with the moveable plates only during the operationof the device. In such an embodiment, the rotary component would beengaged with the posts or curvilinear slots of the moveable plates,depending on the device's design, the moveable plates would be engagedto lock the bone fixation screws into place, and then the rotarycomponent would be removed.

As has been previously discussed, the device can be incorporated into oradded on to any conventional bone fixation plate that uses bone screwsas the means of attaching the plate to the bone. However, the device mayalso be used in other building, construction or home use/repair systemsand devices where screw back-out may be an issue.

The components of the screw back-out prevention device of the currentinvention, and any bone fixation plate into which the device isincorporated, may be formed of any conventional surgical material.Exemplary conventional materials include, for example, titanium andstainless steel. In addition, portions of the device may be made ofmemory metals or smart memory alloys, such as, for example, NiTi. Inparticular, these smart memory alloys are particularly suitable for usein making the retention rings used in some embodiments of the invention.

It should be clear to one of ordinary skill in the art that the drawingsprovided in the current application are only meant to be schematic, andthe size and shape of the slots, posts and other components of the screwback-out prevention device should be designed to ensure that themoveable plates can be positioned to engage or retain the screws forwhich back-out prevention is desired.

While the above description contains many specific embodiments of theinvention, these should not be construed as limitations on the scope ofthe invention, but rather as an example of one embodiment thereof.Accordingly, the scope of the invention should be determined not by theembodiments illustrated, but by the appended claims and theirequivalents.

What is claimed is:
 1. A fastening system comprising: a moveableelement, said moveable element being disposed adjacent to a screw havinga head, an elongated threaded shaft characterized by a longitudinalaxis, and an interlock portion that is configured to be interlockinglyengaged; and a means for repositioning the moveable element into a lockposition whereby the interlock portion of the screw is interlockinglyengaged such that movement of the screw in the direction of the head ofthe screw results in the application of a normal force to the screw in adirection substantially parallel to the longitudinal axis of theelongated threaded shaft thereby restricting said movement of the screw.2. A fastening system comprising: a moveable element having a firstcooperative member disposed on a surface thereof, said moveable elementbeing disposed adjacent to a screw having a head, an elongated threadedshaft characterized by a longitudinal axis, and an interlock portionthat is configured to be interlockingly engaged; a rotary activationelement having a second cooperative member disposed on a surfacethereof; wherein the first cooperative member and the second cooperativemember are designed to interlockingly engage such that rotation of therotary activation element causes the moveable element to advance into alock position whereby the interlock portion of the screw isinterlockingly engaged such that movement of the screw in the directionof the head of the screw results in the application of a normal force onthe screw in a direction substantially parallel to the longitudinal axisof the elongated threaded shaft thereby restricting said movement of thescrew.
 3. The fastening system of claim 2 wherein rotation of the rotaryactivation element causes the moveable element to advance into a lockposition whereby the interlock portion of the screw is interlockinglyengaged by the moveable element.
 4. The fastening system of claim 3wherein the moveable element is configured to directly engage theinterlock portion of the screw when in the lock position.
 5. Thefastening system of claim 3 wherein at least one moveable element isconfigured to engage an interlock portion of a screw through a retentionring when in the lock position.
 6. The fastening system of claim 5wherein the retention ring is integrated into the screw.
 7. Thefastening system of claim 5 wherein the retention ring is integratedinto the screw back-out prevention device.
 8. The fastening system ofclaim 3, wherein the interlock portion of the screw is a slot designedto engage an edge of the moveable element.
 9. The fastening system ofclaim 2, further comprising at least one cam, wherein rotation of therotary activation element causes at least one moveable element toadvance into a lock position whereby the interlock portion of at leastone screw is interlockingly engaged by at least one cam.
 10. A bonefusion system comprising: a bone fusion plate having a hole passingtherethrough; a fixation screw designed to pass through said hole ofsaid bone fusion plate to anchor said plate to a bone; and a screwback-out prevention device, itself comprising: a moveable element, saidmoveable element being disposed adjacent to a screw having a head, anelongated threaded shaft characterized by a longitudinal axis, and aninterlock portion configured to be engaged, and a means forrepositioning the moveable element into a lock position whereby theinterlock portion of the screw is interlockingly engaged such thatmovement of the screw in the direction of the head of the screw resultsin the application of a normal force on the screw in a directionsubstantially parallel to the longitudinal axis of the elongatedthreaded shaft thereby restricting said movement of the screw.
 11. Abone fusion system comprising: a bone fusion plate having a hole passingtherethrough; a fixation screw designed to pass through said hole ofsaid bone fusion plate to anchor said plate to a bone; and a screwback-out prevention device, itself comprising: a moveable element havinga first cooperative member disposed on a surface thereof, said moveableelement being disposed adjacent to a screw having a head, an elongatedthreaded shaft characterized by a longitudinal axis, and an interlockportion configured to be interlockingly engaged, a rotary activationelement having a second cooperative member disposed on a surfacethereof, and wherein the first cooperative member and the secondcooperative member are designed to interlockingly engage such thatrotation of the rotary activation element causes the moveable element toadvance into a lock position whereby the interlock portion of the screwis interlockingly engaged such that movement of the screw in thedirection of the head of the screw results in the application of anormal force on the screw in a direction substantially parallel to thelongitudinal axis of the elongated threaded shaft thereby restrictingsaid movement of the screw.
 12. The bone-fusion system of claim 11,wherein rotation of the rotary activation element causes the moveableelement to advance into a lock position whereby the interlock portion ofthe screw is interlockingly engaged by the moveable element.
 13. Thebone fusion system of claim 12 wherein the moveable element isconfigured to directly engage the interlock portion of the screw when inthe lock position.
 14. The bone fusion system of claim 12 wherein atleast one moveable element is configured to engage the interlock portionof the screw through a retention ring when in the lock position.
 15. Thebone fusion system of claim 12 wherein the retention ring is integratedinto the screw.
 16. The bone fusion system of claim 12 wherein theretention ring is integrated into the screw back-out prevention device.17. The bone fusion system of claim 12 wherein the interlock portion ofthe screw is a slot designed to engage an edge of the moveable element.18. The bone fusion system of claim 12 further comprising at least onecam, wherein rotation of the rotary activation element causes at leastone moveable element to advance into a lock position that causes theinterlock portion of at least one screw to be engaged by at least onecam.